How do I add Floor mouldings with Archipack Pro in Blender 2.8?
You were nearly there.

For automatic flooring and molding etc. you need to click on the Archipack tab and then on the wrench. All that with your walls selected of course.



More generally speaking Stephen hangs out on BlenderArtist if you have questions. I agree that the documentation and the tutorials are lacking in general principles.